Sheet conveying apparatus and image forming apparatus

ABSTRACT

Provided is a sheet conveying apparatus which can improve rippling of a sheet. The sheet conveying apparatus which conveys the sheet includes an electromagnetic hysteresis brake  131  which is configured to apply a tension force in a conveying direction of the sheet and a controller  500 C which performs control on the electromagnetic hysteresis brake  131  to vary a load. The controller  500 C performs control on the load of the electromagnetic hysteresis brake  131  according to information relating to the sheet.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet conveying apparatus whichconveys a sheet and suppresses rippling or curls of the sheet, and animage forming apparatus which includes the sheet conveying apparatus.

2. Description of the Related Art

In the related art, an image forming apparatus which employs anelectrophotographic system develops a latent image formed on aphotosensitive drum serving as an image bearing member so as to be avisible image, and then transfers the visible image (a toner image) ontothe sheet using an electrostatic force. Next, the toner image on thesheet is fixed by heat and pressure, and an image is recorded on thesheet.

As for a fixing apparatus of such an image forming apparatus, a heatroller fixation system is employed. In the heat roller fixation system,a fixing roller is kept in a predetermined temperature using a heatsource such as a heater provided therein, an elastic pressure rollercomes in press contact with the fixing roller to form a nip portion.Further, the toner image is fixed onto the sheet in the nip portion.

However, in a heat fixing process of the fixing apparatus, since theheat and the pressure is applied to the sheet with the toner imagetransferred thereon, moisture is evaporated from the inside of the sheetin process of passing through the nip portion of the fixing apparatus orafter passing through the nip portion of the fixing apparatus. At thistime, a humidity of the sheet is changed due to the heat and a stress isapplied on the sheet due to the pressure. Therefore, a phenomenon calledcurls to bend the sheet or a phenomenon called rippling to make thesheet waved is generated.

Herein, it is considered that sheet paper which is most commonly used asa sheet is observed at a fiber level. The sheet is formed of shortfibrous tissues which are intertwined, and moisture is contained in thefibrous tissue or between the fibrous tissues. Furthermore, there occursan equilibrium state in a state where the fibrous tissue and water arecombined by hydrogen bonding, so that the sheet has smoothness.

However, when the heat and the pressure are applied in the fixingprocess, the fibrous tissues are sheared. In this state, when the heatis added to evaporate the moisture, the hydrogen bonding occurs betweenthe fibrous tissues once more and the sheet is deformed. When the sheetis left as it is, the sheet absorbs moisture from the surroundings, thehydrogen bonding between the fibrous tissues is torn off again andreturns to the original state. However, when the moisture is notabsorbed between some fibrous tissues of the sheet, the deformation ofthe sheet is kept on. As deformation patterns, there are the curls andthe rippling as described above. The curls are generated by an extensiondifference between the front and back surfaces of the sheet, and therippling is generated by the extension difference in the center portionand the end portion of the sheet.

As described above, one of the factors that cause the rippling on theend portion of the sheet lies in a process when the sheet passes throughthe nip portion of the fixing apparatus. For example, in the case of thefixing apparatus which includes a wide nip portion such as a belt fixingsystem, a conveying speed in the nip portion is set to be high for theend portion rather than the center portion in the width directionperpendicular to the conveying direction of the sheet in order toprevent wrinkles of the sheet when the sheet passes through the nipportion. Therefore, in a case where a pulling operation is exerted onthe sheet, the end portion of the sheet is extended in the conveyingdirection compared to a portion near the center portion after passingthrough the nip portion, thereby generating the rippling in the endportion of the sheet.

Further, as described above, one of the factors that cause the ripplingin the end portion of the sheet lies in a process after the sheet passesthrough the nip portion of the fixing apparatus. In a state where thesheets are stacked as a sheet bundle, the end portion of each sheet isin contact with the air, so that moisture frequently goes in and out.When the heat is added to the sheet in the fixing process, the moistureinside the sheet is evaporated, and then the end portion of the sheetrapidly absorbs the moisture, the end portion of the sheet is finallyextended in the conveying direction compared to a portion near thecenter portion, thereby generating the rippling in the end portion ofthe sheet.

Therefore, there is proposed a technology of correcting the deformationof the sheet described above in the related art. U.S. Pat. No. 7,840,173discloses a sheet conveying apparatus in which two decurl portions aredisposed, and the decurl portions include a plurality of roller groupsto correct the curls of the sheet. Then, while the sheet is nipped andconveyed by the roller groups included in the decurl portion, the curlsof the sheet are corrected.

However, a technology disclosed in U.S. Pat. No. 7,840,173 can correctthe curls to make the sheet bent among the deformations of the sheetdescribed above, but it is difficult to sufficiently correct therippling of the sheet caused by a difference in extension occurringbetween the center portion and the end portion of the sheet.

Further, a deformation level of the sheet is different according to abasis weight, a type, an image density, an environmental humidity, andthe like. For this reason, the deformation level is not sufficientlyimproved only by passing the sheet through the decurl portion asdescribed above.

Therefore, the invention is to further develop the technology in therelated art, and it is desirable to provide a sheet conveying apparatuswhich can improve the rippling of the sheet.

Further, it is also desirable to provide a sheet conveying apparatuswhich can improve the rippling of the sheet even under differentconditions such as the basis weight, the type, the image density, andthe environmental humidity.

SUMMARY OF THE INVENTION

It is desirable to provide the following configurations.

A sheet conveying apparatus conveys a sheet with an image formed thereonby an image forming portion, and includes a load portion which isconfigured to apply a tension force in a conveying direction to thesheet and a load controller which performs control on the load portionto vary a load thereof. The load controller performs control on the loadportion to vary the load thereof according to information relating tothe sheet.

Further, an image forming apparatus includes an image forming portionwhich forms a toner image, a fixing portion which heats and fixes thetoner image formed on a sheet by the image forming portion, a loadportion which applies a tension force in a conveying direction to thesheet passed through the fixing portion, and a load controller whichperforms control on the load portion to vary a load. The load controllerperforms control the load of the load portion according to informationrelating to the sheet.

Further, a sheet conveying apparatus which conveys a sheet includes aload portion which applies a tension force of a conveying direction tothe sheet and a load controller which performs control on the loadportion to vary a load. The load controller performs control on the loadportion to apply a load on the sheet according to information relatingto the sheet which is input to the load controller.

According to the invention, since the tension force in the conveyingdirection is applied on the sheet, even when a difference in extensionoccurs between the center portion and the end portion of the sheet, itis possible to reduce the difference and to improve rippling of thesheet.

Furthermore, even in a case where the conditions such as a basis weight,a type, an image density, an environmental humidity are different, it ispossible to apply the tension force on the sheet according to theconditions, and thus the rippling of the sheet can be more effectivelyimproved.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a sheet ripplingcorrection apparatus according to a first embodiment;

FIG. 2 is a cross-sectional view illustrating an electrophotographicprinter according to the first embodiment;

FIG. 3 is a cross-sectional view illustrating a sheet humidifyingapparatus according to the first embodiment;

FIG. 4 is a diagram illustrating the outline of the sheet humidifyingapparatus according to the first embodiment;

FIG. 5 is a block diagram illustrating control of theelectrophotographic printer, the sheet humidifying apparatus, and thesheet pulling and conveying apparatus according to the first embodiment;

FIG. 6 is a flowchart illustrating control of the sheet pulling andconveying apparatus according to the first embodiment;

FIG. 7A is a cross-sectional view illustrating the sheet pulling andconveying apparatus according to the first embodiment;

FIG. 7B is a cross-sectional view illustrating the sheet pulling andconveying apparatus according to the first embodiment;

FIG. 8 is a perspective view illustrating the sheet pulling andconveying apparatus according to the first embodiment;

FIG. 9 is a top view illustrating the sheet pulling and conveyingapparatus according to the first embodiment;

FIG. 10 is a diagram illustrating the outline of the shape of a sheet P;

FIG. 11 is a graph illustrating a relation between an exciting currentof a load portion and a brake torque according to the first embodiment;

FIG. 12A is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 12B is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 12C is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 13A is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 13B is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 13C is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 14A is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 14B is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 15A is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 15B is a table listing states of a sheet obtained by experimentsaccording to the first embodiment;

FIG. 16A is a table listing setting values under various conditions of asheet tension force according to the first embodiment;

FIG. 16B is a table listing setting values under various conditions ofthe sheet tension force according to the first embodiment;

FIG. 17 is a top view illustrating a sheet pulling and conveyingapparatus according to a second embodiment;

FIG. 18 is a flowchart illustrating control of the sheet pulling andconveying apparatus according to the second embodiment; and

FIG. 19 is a cross-sectional view illustrating a sheet ripplingcorrection apparatus according to another embodiment.

DESCRIPTION OF THE EMBODIMENTS

Hereinafter, exemplary embodiments of the invention will be described indetail with reference to the drawings. However, dimension, material,shape, and relative arrangement of components described in the followingembodiments may be appropriately changed according to the configurationor various conditions of the apparatus to which the invention pertains.Therefore, there is no purpose of limiting the scope of the inventiononly to these embodiments, if not otherwise specified.

First Embodiment

An image forming apparatus according to the embodiment will be describedusing FIGS. 1 to 14C.

<Image Forming Apparatus>

FIG. 2 is a cross-sectional view schematically illustrating a colorelectrophotographic printer 500 as an example of the image formingapparatus which is taken along a conveying direction of a sheet. Herein,the color electrophotographic printer will be simply referred to as a“printer”.

The sheet is used to form a toner image thereon. Specific examples ofthe sheet include plain paper, a resin sheet-like medium as a substitutefor the plain paper, thick paper, a medium used for an overheadprojector, and the like.

The printer illustrated in FIG. 2 includes an image forming portion 510of each color of Y (yellow), M (magenta), C (cyan), and Bk (black). Theimage forming portion 510 of each color is used to form a toner image ofeach color on the sheet. The image forming portion 510 of each colorincludes process portions as follows; an electrophotographicphotoconductor (photosensitive drum) 511 serving as an image bearingmember which carries an electrostatic latent image on the surface incorrespondence with each color of Y, M, C, and K, a charging roller 512,a laser scanner 513, and a development device 514. The photosensitivedrum 511 is charged by the charging roller 512 in advance. Then, thephotosensitive drum 511 is exposed to light by the laser scanner 513 andforms a latent image. The latent image is a toner image developed by thedevelopment device 514 and becomes a visible image.

In a primary transfer portion which is formed by the photosensitive drum511 and a primary transfer roller 515, the respective toner imagesformed and carried on the surfaces of the photosensitive drums 511 areprimarily transferred onto an intermediate transfer belt 531 by theprimary transfer roller 515 in a sequentially superimposed manner.

On the other hand, the sheet P is fed out of a sheet cassette 520 one byone and then sent to a registration roller pair 523. The registrationroller pair 523 once stops the sheet P, and corrects skew feeding in acase where the sheet is fed on the skew. Then, the registration rollerpair 523 sends the sheet to a secondary transfer portion between theintermediate transfer belt 531 and a secondary transfer roller 535 insynchronization with the toner images on the intermediate transfer belt531. The color toner images on the intermediate transfer belt aresecondarily transferred onto the sheet P in a lump by the secondarytransfer roller 535 which is a transfer member for example.

Then, as described above, the sheet with the image (the toner image)formed thereon by the image forming portion is conveyed to a fixingapparatus 100. In the fixing apparatus (a fixing portion) 100, the sheetis nipped in a fixing nip portion and the unfixed toner image is appliedwith heat and pressure, so that the toner image is fixed on the sheet.After passing through the fixing apparatus 100, the sheet is sent to asheet rippling correction apparatus 201 by a main discharge roller 540to correct rippling by the sheet rippling correction apparatus 201, andthen discharged onto a discharge tray 565.

<Fixing Apparatus>

Herein, the fixing apparatus will be described. As illustrated in FIG.2, the fixing apparatus 100 includes a fixing roller 110 serving as aheating rotating member and a pressure roller 111 serving as a pressurerotating member. The fixing roller 110 applies heat (which is generatedby a halogen heater (not illustrated) therein) to the toner T on thesheet P while conveying the sheet P in corporation with the pressureroller 111. The fixing roller 110, for example, includes the halogenheater which is built in a metal core made of a cylindrical aluminumtube having an outer diameter of 56 mm and an inner diameter of 50 mm.On the surface of the metal core, for example, an elastic layer made ofsilicon rubber having a thickness of 2 mm and a hardness (Asker C) of45° is coated and the surface of the elastic layer is coated with a PFAor PTFE heat-resistant toner parting layer.

The pressure roller 111 conveys the sheet P in cooperation with thefixing roller 110. The pressure roller 111 also includes, for example, ametal core made of a cylindrical aluminum tube having an outer diameterof 56 mm and an inner diameter of 50 mm. On the surface of the metalcore, for example, an elastic layer made of silicon rubber having athickness of 2 mm and a hardness (Asker C) of 45° is coated and thesurface of the elastic layer is coated with a PFA or PTFE heat-resistanttoner parting layer.

A fixing nip N illustrated in FIG. 2 is formed by the fixing roller 110and the pressure roller 111. The inventors have been performedexperiment in which the sheet P is conveyed at a conveying speed of 300to 500 mm/sec based on conditions as follows: a setting temperature ofthe surface of the fixing roller 110 is 180° C., a setting temperatureof the surface of the pressure roller 111 is 100° C., an environmentaltemperature of 23° C., and an environmental humidity of 50%. Then, thesheet P heated and pressed in the fixing nip N is further applied withheat from the fixing roller 110 which has a temperature higher than thatof the pressure roller 111, and the fibrous tissue on the upper side ofthe sheet P is extended further more than on the lower side thereof. Asa result, curls in the lower direction (hereinafter, referred to aslower curls) are generated. Further, the fibrous tissue of the endportion in a width direction perpendicular to a conveying direction ofthe sheet is extended in the conveying direction further more than thecenter portion, and thus the length of the conveying direction of thesheet becomes different in the end portion and the center portion. As aresult, the rippling is generated in the end portion of the sheet.

<Controller>

As information relating to the sheet, basis weight information of thesheet P in the sheet cassette 520 is input by a user through anoperation panel (operation portion) 570, and the information is sent toa CPU and a memory which included in a controller 500C in the imageforming apparatus illustrated in FIG. 5.

Herein, as information relating to the sheet which can be input from theoperation panel, the basis weight is given as an example in theembodiment, but the invention is not limited thereto. Besides the basisweight, for example, information of types of the sheet (plain paper,coated paper, embossing paper, thin paper, recycled paper, and thelike), size information, and the like are used. The information thusinput relating to the sheet is sent to the controller (load controller)500C as described above.

Further, image density information of the toner image on the sheet Pformed by the image forming portion 510 is sent to the CPU and thememory which are included in the controller (the load controller) 500Cin the image forming apparatus illustrated in FIG. 5.

Further, a temperature and a humidity in the image forming apparatus 500are detected by an environment sensor 500D which is provided over thesheet cassette 520 in the image forming apparatus 500, and thetemperature information and the humidity information are sent to the CPUand the memory which are included in the controller 500C. Theenvironment sensor 500D serves as a humidity detection portion whichdetects environmental humidity.

Herein, the controlling configuration of the entire apparatus will bedescribed using FIG. 5. FIG. 5 is a block diagram illustrating theentire control relation between the image forming apparatus 500 and thesheet rippling correction apparatus 201. The controller 500C of theimage forming apparatus 500 and a controller 201C of the sheet ripplingcorrection apparatus 201 each are a computer system which includes a CPUand a memory and further includes, while not illustrated, a calculationunit, an I/O port, a communication interface, a driving circuit, and thelike.

The controls of the respective controllers described above are performedby predetermined programs, when executed by the respective CPUs, whichare stored in the memory. Further, the respective controllers describedabove are connected to each other through a communication portion COM,and can exchange the information. In addition, the blocks not directlyassociated to the description of the invention are not illustrated inthe drawing.

<Sheet Rippling Correction Apparatus>

Next, as the sheet conveying apparatus which conveys a sheet passedthrough the fixing portion, the sheet rippling correction apparatus 201will be described using FIG. 1. FIG. 1 is a cross-sectional viewillustrating the entire sheet rippling correction apparatus 201. Thesheet P on which a toner image is fixed by heating and pressurizing thetoner image by the fixing apparatus 100 is sent to an input roller pair207 (in a direction of arrow A in FIG. 1) of the sheet ripplingcorrection apparatus 201 by the main discharge roller 540. Further,after the conveying direction is changed by a conveying guide 208 to thevertical direction (a direction of arrow B in FIG. 1), the sheet is sentto a sheet humidifying apparatus 202 serving as a moisture applyingapparatus. Herein, the sheet P is humidified by a humidifying rollerpair 220 and 230 and a humidifying roller pair 221 and 231.

The sheet P discharged out of the sheet humidifying apparatus 202 issuccessively sent to the sheet pulling and conveying apparatus (thesheet conveying apparatus) 101. After being humidified at apredetermined humidity or more by the sheet humidifying apparatus 202,the sheet P passes through the sheet pulling and conveying apparatus101, so that the center portion in the width direction perpendicular tothe conveying direction of the sheet is extended further more than theend portion. Therefore, a difference in length between the end portionof the width direction of the sheet and the center portion of theconveying direction of the sheet is reduced, and thus the rippling isimproved. After the sheet P passes through the sheet pulling andconveying apparatus 101, the conveying direction is changed by aconveying guide 209 to a direction of arrow C in FIG. 1, and thendischarged to the outside of the sheet rippling correction apparatus 201by a discharge roller pair 210 and is stacked on the discharge tray 565.

In FIG. 1, a reservoir 204 contains humidification liquid LQ1 used forhumidifying the sheet P. The humidification liquid LQ1 contained in thereservoir 204 is supplied in a direction of arrow D in FIG. 1 by a pump206 through a water pipe 205 toward a water tank 240 which is providedin the sheet humidifying apparatus 202. Humidification liquid LQ2 issupplied into the water tank 240. The humidification liquid LQ1 and LQ2contains water as a main component.

<Sheet Humidifying Apparatus>

Next, the sheet humidifying apparatus 202 in the sheet ripplingcorrection apparatus will be described using FIGS. 3 and 4. FIG. 3 is across-sectional view illustrating the entire sheet humidifying apparatus202, and FIG. 4 is a diagram illustrating the outline of the sheethumidifying apparatus 202.

The sheet P sent in the same direction of arrow B in FIG. 3 as that ofarrow B in FIG. 1 is introduced into an inlet guide 250 and sent to thenip portion of a first humidifying roller pair 220 and 230, and istransferred by the humidification liquid LQ2 and humidified. Next, thesheet P is further humidified by transferring the humidification liquidLQ2 again onto the surface in process of passing through the nip portionof a second humidifying roller pair 221 and 231.

Next, the sheet P passed through the nip portion of the secondhumidifying roller pair 221 and 231 passes through a conveying rollerpair 222 and 232 and sent to the sheet pulling and conveying apparatus101 through a humidifying portion discharge guide 251.

The respective humidifying rollers of the first humidifying roller pair220 and 230 and the second humidifying roller pair 221 and 231 are allelastic rollers in which a solid rubber layer containing NBR and siliconas main components is formed in the surface of a shaft core made of ametal rigid body such as stainless steel.

Water feeding rollers 223, 224, 225, 233, 234, and 235 sequentiallysupply the humidification liquid LQ2 in the water tank 240 to therespective humidifying rollers of the first humidifying roller pair 220and 230 and the second humidifying roller pair 221 and 231. The waterfeeding rollers 223, 224, 225, 233, 234, and 235 are the elastic rollersin which a material of which the surface has hydrophilicity to holdwater on the surface of the shaft core made of the metal rigid body suchas the stainless steel; for example, a solid rubber layer containing NBRas a main component is formed on the surface of the shaft core. As thesolid rubber layer, a metal or a hydrophilic-processed resin may beused.

The water feeding roller 223 comes into contact with both thehumidifying roller 220 and the humidifying roller 221 among therespective humidifying rollers of the first humidifying roller pair 220and 230 and the second humidifying roller pair 221 and 231. Further,since the water feeding roller 224 comes into contact with the waterfeeding roller 223 and the water feeding roller 225 comes into contactwith the water feeding roller 224, a water path is formed from theinside of the water tank 240 toward the humidifying roller 220 and thehumidifying roller 221.

The water feeding roller 233 comes into contact with both thehumidifying roller 230 and the humidifying roller 231 among therespective humidifying rollers of the first humidifying roller pair 220and 230 and the second humidifying roller pair 221 and 231. Further,since the water feeding roller 234 comes into contact with the waterfeeding roller 233 and the water feeding roller 235 comes into contactwith the water feeding roller 234, a water path is formed from theinside of the water tank 240 toward the humidifying roller 230 and thehumidifying roller 231.

Regulating rollers 226 and 236 are configured to regulate a water amountto be supplied to the respective humidifying rollers of the firsthumidifying roller pair 220 and 230 and the second humidifying rollerpair 221 and 231. The regulating rollers 226 and 236 are rollers whichare subjected to coating treatment with nickel, chrome, or the like onthe surface of the shaft core made of the metal rigid body such as thestainless steel.

The regulating roller 226 comes into contact with the water feedingroller 224, and the regulating roller 236 comes into contact with thewater feeding roller 234 so as to suppress the amount of thehumidification liquid held on the surface of each solid rubber layer andto regulate the amount of water to be supplied to the sheet P. In otherwords, the regulating rollers 226 and 236 come into press contact withthe solid rubber layers of the water feeding rollers 224 and 234 tocause deformation, and squeeze the humidification liquid contained inthe surface. Therefore, the sheet P is humidified at an optimalhumidity, and thus the extending effect is accelerated by theabove-mentioned sheet pulling and conveying apparatus 101.

As illustrated in FIG. 3, the respective rollers of the firsthumidifying roller pair 220 and 230 and the second humidifying rollerpair 221 and 231, the water feeding rollers 223, 224, 225, 233, 234, and235, the regulating rollers 226 and 236, and the conveying roller pair222 and 232 are bisymmetrically arranged with the sheet P interposedtherebetween and each rotate in the respective directions of arrows.Therefore, both faces of the sheet P are uniformly humidified withwater.

Among the above-mentioned rollers, the end portions of the humidifyingrollers 230 and 231 and the conveying roller 232 disposed on the leftside with respect to the sheet P are respectively fixed to driving gears260, 261, and 262 as illustrated in FIG. 4 and a rotation driving forceis transferred by driving motors (not illustrated). The other rollersare rotatably driven by a driving force transferred from the respectivesurfaces of the humidifying rollers 230 and 231 and the conveying roller232.

By supplying moisture to the sheet using the humidifying roller pairwhich humidifies the above-mentioned sheet P, water content can beincreased up to a level required for accelerating the extensionaccording to a tension load on the center portion in the width directionof the sheet P by the sheet pulling and conveying apparatus 101.

<Sheet Pulling and Conveying Apparatus>

Next, the sheet pulling and conveying apparatus 101 in the sheetrippling correction apparatus will be described using FIGS. 7A to 9.FIGS. 7A and 7B are cross-sectional views illustrating the sheet pullingand conveying apparatus, FIG. 8 is a perspective view illustrating thesheet pulling and conveying apparatus, and FIG. 9 is a top viewillustrating the sheet pulling and conveying apparatus.

The sheet pulling and conveying apparatus 101 conveys the sheet P whilenipping the sheet P using a first roller A104 and a first roller B105which are included in a first roller pair illustrated in FIGS. 7A and7B, and a second roller A106 and a second roller B107 which are includedin a second roller pair on the downstream in the conveying direction ofthe first roller pair. The sheet pulling and conveying apparatus 101applies a tension force to the sheet P in order to extend the centerportion of the width direction of the sheet to the conveying directionwhile nipping and conveying the sheet. Then, the sheet P is guidedbetween a discharge upper guide 117 and a discharge lower guide 118, anddischarged to the outside of the sheet pulling and conveying apparatus101.

As illustrated in FIG. 8, the first roller A104, the first roller B105,the second roller A106, and the second roller B107 have elastic rubbers104 b, 105 b, 106 b, and 107 b such as silicon, NBR, and EPDM. Theelastic rubbers 104 b, 105 b, 106 b, and 107 b are formed on thesurfaces of roller shafts 104 a, 105 a, 106 a, and 107 a made of a highrigidity material such as stainless steel and iron. Herein, all theouter diameters φ of the elastic rubbers 104 b, 105 b, 106 b, and 107 bare set to 20 mm. Further, as illustrated in FIG. 8, the elastic rubbers105 b and 107 b of the first roller B105 and the second roller B107 areformed in a region corresponding to the length L1 of the center portionin the width direction of the sheet so as to be uniformly disposed withrespect to a sheet passing center. In other words, the outer diametersof the first roller B105 and the second roller B107 are provided suchthat the center region in the width direction perpendicular to theconveying direction of the sheet has an outer diameter larger than thatof the end region. Herein, the sheet passing center is a position of thecenter in the width direction as a reference when the sheet is conveyed.The length L1 is shorter than the length of the sheet P in the widthdirection which may become a problem causing the rippling illustrated inFIG. 10. Herein, the length L1 is set to 100 mm (L1=100 mm).

Further, a conveying upper guide 114 and a conveying lower guide 115serving as guide members to guide the sheet are provided between nipportions of the first roller pair and the second roller pair, and thedistance between the nip portions is set to 25 mm.

The first roller A104 and the second roller A106 are provided such thatboth ends of the roller shafts 104 a and 106 a are supported to an upperplate 119 through gears (not illustrated).

The first roller B105 is provided such that both ends of the rollershaft 105 a are supported to a first pressure plate 113 through a gear(not illustrated). The first pressure plate 113 is rotatably supportedto a lower plate 120 through a first rotation shaft (not illustrated),and the bottom surface is urged by a first pressure spring 109.Therefore, the first roller B105 is pressurized to the first rollerA104, so that the nip portion is formed.

The second roller B107 is provided such that both ends of the rollershaft 107 a are supported to a second pressure plate 112 through a gear(not illustrated). The second pressure plate 112 is rotatably supportedto the lower plate 120 through a second rotation shaft 122, and thebottom surface is urged by a second pressure spring 108. Therefore, thesecond roller B107 is pressurized to the second roller A106, so that thenip portion is formed.

A reflective light type of sheet sensor 103 is disposed in an entrancelower guide 121 to detect whether the sheet P is arrived at.

FIG. 9 is a top view for describing the driving of the first roller A104and the second roller A106.

In the drawing, the CPU is a controller which controls the operations ofan electromagnetic clutch CL serving as a clutch portion (drivingcontroller) and a driving motor M serving as a driving portion.

As illustrated in FIG. 9, the driving gear 104G1 is held and fixed onone end of the first roller A104. The first roller A104 is rotated byreceiving a rotation driving force through driving transmission gears123, 124, and 125 and a clutch gear CLG from a motor gear MG of thedriving motor M serving as a driving source (the driving portion). Thefirst roller B105 pressurized by the first roller A104 is rotatablydriven according to the rotation of the first roller A104.

The driving gear 106G is held and fixed on one end of the second rollerA106. The second roller A106 is rotated by receiving a rotation drivingforce through driving transmission gears 126, 127, 128, and 129 from themotor gear MG of the driving motor M serving as the driving source (thedriving portion). The second roller B107 pressurized by the secondroller A106 is rotatably driven according to the rotation of the secondroller A106.

The clutch gear CLG is fixed on the electromagnetic clutch CL. While theelectromagnetic clutch CL is powered on, the driving force between theclutch gear CLG and the driving transmission gear 124 is transferredthrough a clutch shaft 132, and the first roller A104 rotates. On theother hand, when the electromagnetic clutch CL is not powered on, thedriving force between the clutch gear CLG and the driving transmissiongear 124 is not transferred, the driving force of the driving motor M isnot transferred to the driving gear 104G, and thus the first roller A104does not rotate.

Further, the driving gear 104G2 is fixed on the other end of the firstroller A104. The driving gear 104G2 is connected to a load portion 131such as an electromagnetic powder brake and an electromagnetichysteresis brake through a driving transmission gear 130.

The load portion 131 applies a tension force to the sheet in theconveying direction, and can change a brake torque according to anexciting current value. In addition, in the following description, theelectromagnetic hysteresis brake (electromagnetic brake) will beexemplified as the load portion. FIG. 11 illustrates a relation betweenan exciting current of the electromagnetic hysteresis brake used in theembodiment and the brake torque. As illustrated in FIG. 11, the excitingcurrent and the brake torque are proportional to each other, and theelectromagnetic hysteresis brake can be varied to be a predeterminedbrake torque by changing the exciting current as needed.

As described in the following, the transmission of a driving force tothe first roller pair is disconnected at a timing point when the sheet Pis arrived at the nip portion of the second roller pair. After thetransmission of the driving force to the first roller pair isdisconnected, the first roller pair generates a load on the conveyanceof the sheet P by the second roller pair. Therefore, when the sheet P isnipped between the first roller pair and the second roller pair, thesheet P is conveyed while a tension force (a tension force in theconveying direction of the sheet) is generated in the sheet P conveyedby the second roller pair. Then, the tension force loaded on the sheet Pis also changed according to a magnitude of the brake torque generatedby the load portion 131.

In the embodiment, when the brake torque is 100%, the tension forceloaded on the sheet is set to be 8 kgf. In the embodiment, when thetension force becomes 8 kgf or more, the load is increased, so that theroller slips over the sheet in the nip portion of the second roller pair(106 and 107).

FIG. 6 illustrates a flowchart relating to driving control according tothe embodiment, and FIG. 5 illustrates a block diagram relating to thedriving control according to the embodiment. FIGS. 7A and 7B illustratefront cross-sectional views of the sheet pulling and conveying apparatus101 for describing the driving control according to the embodiment. FIG.7A is the front cross-sectional view during a period of time from 0 to Xmsec after the sheet sensor is turned on, and FIG. 7B is the frontcross-sectional view at the time of X msec after the sheet sensor isturned on.

When a sheet passing job signal is input to the CPU (the controller)(S6-1), the basis weight, the type, the image density, and the humidityinformation are confirmed as information relating to the sheet (S6-2).Then, the exciting current is set such that the brake torque necessaryfor the electromagnetic hysteresis brake 131 serving as the load portionbecomes a brake torque corresponding to a predetermined sheet tensionforce according to the information relating to the above sheet (S6-3).The predetermined sheet tension force according to the informationrelating to the sheet will be described below in detail.

Then, the driving motor M is powered on (S6-4), and the current alsostarts to flow to the electromagnetic hysteresis brake 131 (S6-5). Atthe same time, the electromagnetic clutch CL is powered on and the sheetstarts to be passed (S6-6). As a result, the driving force of thedriving motor M is transferred to the driving gears 104G1 and 106Gthrough the driving transmission gear, so that the first roller A104 andthe second roller A106 are rotated.

Then, the sheet P is guided to the entrance lower guide 121 in the sheetpulling and conveying apparatus 101, and when an ON signal of the sheetsensor 103 is confirmed (S6-7), the electromagnetic clutch CL is poweredoff after X msec (S6-8). When the electromagnetic clutch CL is poweredoff, the driving force to the first roller A104 is released. The valueof time X is a time taken until the leading end of the sheet P is justinterposed in the nip portion of the second roller pair after the sheetsensor 103 is turned on, and a conveying speed of the sheet P and adistance from the sheet sensor 103 to the nip portion of the secondroller pair are determined. Herein, since the conveying speed of thesheet P is set to 300 mm/s, and a distance from the sheet sensor 103 tothe nip portion of the second roller pair is set to 45 mm, time X is setto 160 msec.

As illustrated in FIG. 7A, since the electromagnetic clutch CL comes tobe the ON state during a period of time from 0 to X msec after the sheetsensor is turned on, the driving force is transferred to the firstroller A104, and the sheet P is conveyed by the driving of the firstroller A104. On the other hand, as illustrated in FIG. 7B, the leadingend of the sheet P is just arrived at the nip portion of the secondroller pair at the time point of X msec after the sheet sensor is turnedon, and the sheet P is conveyed by the driving force of the secondroller A106. At the same time, since the electromagnetic clutch CL comesto be the OFF state and the driving force is not transferred to thefirst roller A104, the first roller pair is rotatably driven to thesheet conveyed by the second roller pair.

When being rotatably driven, the first roller A104 is connected to theload portion 131 through the driving gear 104G2 and the drivingtransmission gear 130, so that a torque load is generated in order torotate the first roller A104 of which the driving force is released. Asa result, in FIG. 7B, the sheet P is conveyed while a tension force inthe conveying direction of the sheet is generated on the sheet P betweenthe first roller pair and the second roller pair. In the embodiment, theload torque (the brake torque) of the load portion (the electromagnetichysteresis brake) 131 is set such that the tension force applied on thesheet P becomes a predetermined tension force based on informationrelating to the sheet such as the sheet type, the image density, and thehumidity information.

Further, in the embodiment, as illustrated in FIG. 8, the nip portionbetween the first roller pair and the second roller pair is formed in aregion corresponding to the length L1 (100 mm here) of the centerportion in the width direction of the sheet so as to be uniformlydisposed with respect to a sheet passing center. With thisconfiguration, only the center portion in the width direction of thesheet P between the first roller pair and the second roller pair isapplied with the predetermined tension force over a range from theleading end of the trailing end. Herein, the description has been madeabout that the tension force of the conveying direction applied to thesheet is only to the center portion in the width direction, but theinvention is not limited thereto. The tension force of the conveyingdirection applied to the sheet may be set to be large in the center areain the width direction compared to the end areas such that a differencein extension between the center portion and the end portions in thewidth direction comes to be reduced.

Then, it is determined whether the job is ended (S6-9), and in a casewhere the job is ended, the flow from S6-2 is repeatedly performed basedon the basis weight, the type, the image density, and the humidityinformation of the next job which is subsequently input. When the job isended, the driving motor M and the electromagnetic hysteresis brake 131are turned off and the sheet passing is ended (S6-10), and then the jobis ended (S6-11).

The curls generated in the sheet P illustrated in FIG. 10, a feature ofthe rippling shape in the end portion, and a measurement method will bedescribed. The rippling is measured in a state where the sheet P isplaced on a measurement plate 700. Herein, the length of the end portionof the sheet P in the conveying direction is set to L edge [mm], and thelength of the center portion is set to L center [mm].

Further, an upper side or a lower side of the sheet P illustrated inFIG. 10, that is, bend shapes Pwave generated in the end portions in thewidth direction perpendicular to the conveying direction are referred toas an end portion rippling, and one of these bend shapes having amaximum gap X max on the measurement plate 700 is a target forevaluating a rippling amount.

The inventors have been performs experiments for confirming effects ofthe sheet pulling and conveying apparatus 101 in the embodiment, and theexemplary results are shown in FIGS. 12A to 15. Specifically, the lengthL edge [mm] of the end portion, the length L center [mm] of the centerportion, and a maximum rippling amount X max [mm] of the sheet P weremeasured under various conditions.

A usual environment (a temperature of 23° C. and a humidity of 50%) wasset as a basic experiment condition in this experiment, and the densityof the toner image fixed on the sheet was set to be less than 50% of thetoner density. Further, in a case where the sheet passed through thesheet humidifying apparatus 202 of the embodiment under the above basicexperiment condition, the sheet humidity immediately after the passingwas set to 7% or more.

The inventors measured the humidity contained in the sheet in theembodiment using the sheet P immediately after the sheet P passedthrough the sheet rippling correction apparatus 201 and discharged ontothe discharge tray 565. Herein, a microwave type of sheet humiditymeasuring meter was used.

Further, herein, a condition for the achievement of rippling correctionwas set to a case where a maximum rippling amount of the sheet waslowered down to 1.0 mm or less.

FIGS. 12A to 12C show the results obtained when the plain paper having abasis weight of 81 gsm as information relating to the sheet besides thebasic experiment condition is passed. FIG. 12A shows the case of oneimage forming apparatus (no sheet passing in the sheet ripplingcorrection apparatus), and FIGS. 12B and 12C show a case where the sheetis passed in the sheet rippling correction apparatus 201. The sheettension force of FIG. 12B was set to 1 kgf, and the sheet tension forceof FIG. 12C was set to 3 kgf.

In the case of FIG. 12A (no tension force), an extension amount of thelength of the end portion of the sheet was 0.6 mm, an extension amountof the length of the center portion of the sheet was 0.0 mm, and themaximum rippling amount was 3.3 mm.

Compared to the case of no tension force, the case of the tension forcehas obtained the following results.

In a case where FIG. 12B (a tension force of 1 kgf), the extensionamount of the length of the end portion of the sheet was 0.6 mm, theextension amount of the length of the center portion of the sheet was0.3 mm, and the maximum rippling amount was 1.7 mm. In contrast, in thecase of FIG. 12C (a sheet tension force of 3 kgf), the extension amountsof the lengths of the end and center portions of the sheet were 0.6 mm,and the maximum rippling amount was 1.0 mm.

Therefore, in the basic experiment condition, it can be known that inthe case of the plain paper having a basis weight of 81 gsm, there was adifference between the extension amounts of the center portion of thesheet under the sheet tension forces of 1 kgf and 3 kgf. When the sheettension force was 3 kgf or more, a difference in length of the endportion and the center portion was able to be made to 0 mm, and themaximum amount of the sheet rippling was able to be improved from 3.3 mmof FIG. 12A to 1.0 mm of FIG. 12C. In addition, in FIGS. 12B and 12C,the humidity of the sheet was 8.0%.

FIGS. 13A to 13C show the results obtained when the thick paper having abasis weight of 157 gsm as information relating to the sheet besides thebasic experiment condition is passed. FIG. 13A shows the case of oneimage forming apparatus (no sheet passing in the sheet ripplingcorrection apparatus), and FIGS. 13B and 13C show a case where the sheetis passed in the sheet rippling correction apparatus 201. The sheettension force of FIG. 13B was set to 3 kgf, and the sheet tension forceof FIG. 13C was set to 4 kgf.

In the case of FIG. 13A (no tension force), an extension amount of thelength of the end portion of the sheet was 0.5 mm, an extension amountof the length of the center portion of the sheet was 0.0 mm, and themaximum rippling amount was 2.5 mm.

Compared to the case of no tension force, the case of the tension forcehas obtained the following results.

In a case where FIG. 13B (a tension force of 3 kgf), the extensionamount of the length of the end portion of the sheet was 0.5 mm, theextension amount of the length of the center portion of the sheet was0.3 mm, and the maximum rippling amount was 1.7 mm. In contrast, in thecase of FIG. 13C (a sheet tension force of 4 kgf), the extension amountsof the lengths of the end and center portions of the sheet both were 0.5mm, and the maximum rippling amount was 1.0 mm.

Therefore, in the basic experiment condition, when the sheet tensionforce was 4 kgf or more in the case of the thick paper having a basisweight of 157 gsm, a difference in length of the end portion and thecenter portion was able to be made to 0 mm. Further, the maximum amountof the sheet rippling was able to be improved from 2.5 mm of FIG. 13A to1.0 mm of FIG. 13C. As in the case of the thick paper illustrated inFIGS. 13A to 13C, when the basis weight of the sheet is increased, thesheet thickness becomes thick, so that a higher sheet tension force isrequired compared to the plain paper illustrated in FIGS. 12A to 12C. Inaddition, in FIGS. 13B and 13C, the humidity of the sheet was 7.7%.

FIGS. 14A and 14B show the results obtained when a sheet density lessthan 50% in the basic experiment condition is set to 50% or more asinformation relating to the sheet, and a thick paper having a basisweight of 157 gsm, on which an image is fixed at a high image density(the sheet density equal to or more than 50%), is passed.

FIGS. 14A and 14B show a case where the sheet is passed in the sheetrippling correction apparatus 201. The sheet tension force of FIG. 14Awas set to 4 kgf, and the sheet tension force of FIG. 14B was set to 5kgf.

In the case of FIG. 14A (a tension force of 4 kgf), the extension amountof the length of the end portion of the sheet was 0.5 mm, the extensionamount of the length of the center portion of the sheet was 0.3 mm, andthe maximum amount of the rippling was 1.7 mm. In contrast, in the caseof FIG. 14B (a sheet tension force of 5 kgf), the extension amounts ofthe lengths of the end and center portions of the sheet both were 0.5mm, and the maximum amount of the rippling was 1.0 mm.

Therefore, in the high image density (the sheet density equal to or morethan 50%), when the sheet tension force was 5 kgf or more in the case ofthe thick paper having a basis weight of 157 gsm, a difference in lengthof the center portion of the sheet was able to be made to 0 mm, and themaximum amount of the sheet rippling was able to be improved to 1.0 mm.

In FIGS. 14A and 14B, since an influence of the humidity permeating thesheet on the toner image is reduced in the case of a high density image(the sheet density equal to or more than 50%), the sheet tension force(compared to a low density image) is necessarily increased. In addition,in FIGS. 14B and 14C, the humidity of the sheet was 7.3%.

FIGS. 15A and 15B show the results obtained when a thick paper having abasis weight of 157 gsm is passed, in which the thick paper is fixedwith a toner image under an environment of a low humidity less than 20%such as a humidity of about 5% and a high image density (the sheetdensity equal to or more than 50%). In addition, in the embodiment, theenvironment of a humidity less than 20% is referred to as a low humidityenvironment, and the environment of a humidity equal to or more than 20%is referred to as a usual environment.

FIGS. 15A and 15B show a case where the sheet is passed in the sheetrippling correction apparatus 201. The sheet tension force of FIG. 15Ais set to 5 kgf, and the sheet tension force of FIG. 15B is set to 6kgf.

In the case of FIG. 15A (a tension force of 5 kgf), the extension amountof the length of the end portion of the sheet was 0.5 mm, the extensionamount of the length of the center portion of the sheet was 0.3 mm, andthe maximum amount of the rippling was 1.7 mm. In contrast, in the caseof FIG. 15B (a sheet tension force of 6 kgf), the extension amounts ofthe lengths of the end and center portions of the sheet both were 0.5mm, and the maximum amount of the rippling was 1.0 mm.

Therefore, in the low humidity environment (a humidity less than 20%)and the high image density (the sheet density equal to or more than50%), when the sheet tension force was 6 kgf or more in the case of thethick paper having a basis weight of 157 gsm, a difference in length ofthe center portion of the sheet was able to be made to 0 mm. Further,the maximum amount of the sheet rippling was able to be improved to 1.0mm.

In FIGS. 15A and 15B, since the humidity permeating the sheet is reducedcompared to that in the usual humidity environment in the case of thelow humidity environment (the humidity less than 20%) and the highdensity image (the sheet density equal to more than 50%), the sheettension force is necessarily increased. In addition, in FIGS. 15A and15B, the humidity of the sheet was 7.0%.

In FIG. 16, the setting values of the sheet tension force in theembodiment which are obtained from the experiments under variousconditions including the above experiments are listed. In the settingvalues of FIG. 16, as a result of repeatedly performing experimentsaccording to various conditions, it was confirmed that the ripplingamount in all the conditions was 1.0 mm or less.

In other words, FIG. 16A shows a case where the basis weight is lessthan 90 gsm, a case where the basis weight is 90 gsm or more, a casewhere the image density is less than 50%, and a case where the imagedensity is 50% or more under the usual environment (a humidity equal toor more than 20%). Further, FIG. 16B shows a case where the basis weightis less than 90 gsm, a case where the basis weight is 90 gsm or more, acase where the image density is less than 50%, and a case where theimage density is 50% or more under the low humidity environment (thehumidity less than 20%).

For example, in a case where the image density is less than 50% underthe usual environment (the humidity equal to or more than 20%), when thebasis weight is different, the controller 500C serving as the loadcontroller performs control as illustrated in FIG. 16A. In other words,the controller 500C causes the load portion 131 to apply a tension force(a first tension force) of 3 kgf as a first load on the sheet having abasis weight less than 90 gsm (a first basis weight). On the other hand,the controller causes the load portion 131 to apply a tension force (asecond tension force) of 4 kgf larger than the first tension force as asecond load larger than the first load on the sheet having a basisweight (a second basis weight) of 90 gsm or more, which is larger thanthe first basis weight. Therefore, the rippling of the sheet can beimproved to be 1.0 mm or less. In addition, when a load applied on thesheet having a low basis weight by the load portion 131 is too large,there is a possibility to tear the sheet, so that it is undesirable.

Further, for example, in a case where the basis weight of the sheet isless than 90 gsm under the usual environment (the humidity equal to ormore than 20%), when the image density is different, the controller 500Cserving as the load controller performs control as illustrated in FIG.16A. In other words, the controller 500C causes the load portion 131 toapply a tension force (a first tension force) of 3 kgf as a first loadon the sheet having an image density less than 50% (a first density). Onthe other hand, the controller causes the load portion 131 to apply atension force (a second tension force) of 4 kgf larger than the firsttension force as a second load larger than the first load on the sheethaving an image density (a second density) of 50% or more, which islarger than the first density. Therefore, the rippling of the sheet canbe improved to be 1.0 mm or less.

Further, for example, in a case where the image density is less than 50%and the basis weight of the sheet is less than 90 gsm, when anenvironmental humidity is different, the controller 500C serving as theload controller performs control as illustrated in FIGS. 16A and 16B. Inother words, the controller 500C causes the load portion 131 to apply atension force (a first tension force) of 3 kgf as a first load on thesheet when the humidity is 20% or more (a first humidity). On the otherhand, the controller causes the load portion 131 to apply a tensionforce (a second tension force) of 4 kgf larger than the first tensionforce as a second load larger than the first load on the sheet when thehumidity is less than 20% (a second humidity) lower than the firsthumidity. Therefore, the rippling of the sheet can be improved to be 1.0mm or less.

In the embodiment, when the sheet tension force is large, a load torqueapplied on the motors and the gears is increased and temperature andnoises are increased, so that the sheet tension force is designed to beoptimized under various conditions. In a humidity under the usualenvironment where the sheet is passed at a highest frequency, it ispossible to significantly improve the rippling amount while lowering thebrake torque when a plain paper (a basis weight less than 90 gsm)containing a low density image (the toner image less than 50%) ispassed.

As described above, the sheet is coated with moisture to be humidifiedat a predetermined humidity or more and then passed in the sheet pullingand conveying apparatus where the center portion in the width directionof the sheet is extended in the conveying direction. Therefore, adifference in length in the conveying direction of the sheet in thecenter portion and the end portion in the width direction of the sheetis reduced, and thus the rippling can be improved.

Furthermore, it is possible to more effectively improve the rippling ofthe sheet by selecting (varying) the sheet tension force on extendingaccording to information (the basis weight, the sheet type, the density,and the environmental humidity) relating to the sheet.

Second Embodiment

The configuration of the sheet pulling and conveying apparatus accordingto the embodiment will be described using FIGS. 17 to 19. FIG. 17 is atop view illustrating the sheet pulling and conveying apparatus 101 usedin the embodiment. The embodiment is different from the first embodimentonly in the sheet pulling and conveying apparatus 101, and thus theconfigurations overlapped with the first embodiment will not berepeated.

In the above-mentioned embodiment, as the load portion, theelectromagnetic hysteresis brake (the electromagnetic brake) isexemplified in which the brake torque can be changed as the excitingcurrent is changed. In this regard, as the load portion, the embodimentdescribes a configuration in which a plurality of torque limiters TL1 toTL3 is connected to the driving gear 104G2 of the first roller A104through clutches CL2 to CL4. Then, the load controller performs controlsuch that the clutches CL2 to CL4 are selectively connected ordisconnected so as to vary a load torque (in the following description,by 1 kgf in a range of 3 kgf to 6 kgf) by the torque limiters TL1 toTL3. Hereinafter, the description will be made in detail.

As illustrated in FIG. 17, a first torque limiter TL1 is connected tothe driving gear 104G2 of the first roller A104 through anelectromagnetic clutch CL2, a clutch gear CLG2, and a torque limitergear TLG1. Further, a third torque limiter TL3 is connected to thedriving gear 104G2 of the first roller A104 through an electromagneticclutch CL4, a clutch gear CLG4, and a torque limiter gear TLG3.

Furthermore, a second torque limiter TL2 is connected in series to theclutch gear CLG2 which is connected to the driving gear 104G2 of thefirst roller A104, through an electromagnetic clutch CL3, a clutch gearCLG3, and a torque limiter gear TLG2.

In the embodiment, the setting value of the first torque limiter TL1 isset to make the sheet tension force become 3 kgf. Further, the settingvalue of the second torque limiter TL2 is set to make the sheet tensionforce become 2 kgf. Further, the setting value of the third torquelimiter TL3 is set to make the sheet tension force become 1 kgf.

With the above configuration, in a case where the electromagneticclutches CL2, CL3, and CL4 are turned off, the connection of therespective torque limiters TL1, TL2, and TL3 to the driving gear 104G2of the first roller A104 is released, and thus the tension forcedisappears.

In contrast, in a case where the electromagnetic clutch CL2 is turnedon, the electromagnetic clutch CL3 is turned off, and theelectromagnetic clutch CL4 is turned off, the first torque limiter TL1is connected to the driving gear 104G2 of the first roller A104 by theelectromagnetic clutch CL2. Therefore, the load torque (3 kgf) of thefirst torque limiter TL1 is applied to the first roller A104, and thesheet tension force becomes 3 kgf.

Further, in a case where the electromagnetic clutch CL2 is turned on,the electromagnetic clutch CL3 is turned off, and the electromagneticclutch CL4 is turned on, the first torque limiter TL1 and the thirdtorque limiter TL3 are connected to the driving gear 104G2 of the firstroller A104 by the respective electromagnetic clutches CL2 and CL4.Therefore, the load torque (3 kgf) of the first torque limiter TL1 andthe load torque (1 kgf) of the third torque limiter TL3 are applied tothe first roller A104, and the sheet tension force becomes 4 kgf.

Similarly, in a case where the electromagnetic clutch CL2 is turned on,the electromagnetic clutch CL3 is turned on, and the electromagneticclutch CL4 is turned off, the first torque limiter TL1 and the secondtorque limiter TL2 are connected to the driving gear 104G2 of the firstroller A104 by the respective electromagnetic clutches CL2 and CL3.Therefore, the load torque (3 kgf) of the first torque limiter TL1 andthe load torque (2 kgf) of the second torque limiter TL2 are applied tothe first roller A104, and the sheet tension force becomes 5 kgf.

Similarly, in a case where the electromagnetic clutch CL2 is turned on,the electromagnetic clutch CL3 is turned on, and the electromagneticclutch CL4 is turned on, all the torque limiters TL1, TL2, and TL3 areconnected to the driving gear 104G2 of the first roller A104 by all theelectromagnetic clutches CL2, CL3, and CL4. Therefore, the load torque(3 kgf) of the first torque limiter TL1, the load torque (2 kgf) of thesecond torque limiter TL2, and the load torque (1 kgf) of the thirdtorque limiter TL3 are applied to the first roller A104, and the sheettension force becomes 6 kgf.

With the above configurations and controls, it is possible to vary thesheet tension force by 1 kgf in a range of 3 to 6 kgf. The predeterminedsheet tension force of the embodiment is set as listed in FIG. 16similarly to the first embodiment.

FIG. 18 is a flowchart illustrating the control according to theembodiment. The description on the same portions of the control as thoseof the first embodiment illustrated in FIG. 6 will not be repeated.

When a sheet passing job signal is input to the CPU (the controller)(S18-1), the basis weight, the type, the image density, and the humidityinformation are confirmed as information relating to the sheet (S18-2).Then, ON/OFF of the electromagnetic clutches CL2, CL3, and CL4 are setsuch that the brake torque necessary for the torque limiters TL1, TL2,and TL3 serving as the load portion becomes a brake torque correspondingto a predetermined sheet tension force according to the informationrelating to the above sheet (S18-3). In addition, the predeterminedsheet tension force according to the information relating to the sheetis the same as that of the first embodiment described above, and on thebasis of the predetermined sheet tension force, the electromagneticclutches CL2, CL3, and CL4 are controlled to be powered on/off asdescribed above.

Then, the driving motor M is powered on (S18-4), the electromagneticclutches CL2, CL3, and CL4 is powered on or off (S18-5). At the sametime, the electromagnetic clutch CL is also powered on and the sheetstarts to be passed (S18-6). As a result, the driving force of thedriving motor M is transferred to the driving gears 104G1 and 106Gthrough the driving transmission gear, so that the first roller A104 andthe second roller A106 are rotated.

Then, the sheet P is guided to the entrance lower guide 121 in the sheetpulling and conveying apparatus 101, and when an ON signal of the sheetsensor 103 is confirmed (S18-7), the electromagnetic clutch CL ispowered off after X msec (S18-8). When the electromagnetic clutch CL ispowered off, similarly to the first embodiment, the tension force thusset is applied to the sheet, and the sheet is extended.

Further, even in the embodiment similarly to the first embodiment, asillustrated in FIG. 8, the nip portion between the first roller pair andthe second roller pair is formed in a region corresponding to the lengthL1 (herein, a width of 100 mm) of the center portion in the widthdirection of the sheet so as to be uniformly disposed with respect to asheet passing center. With this configuration, only the center portionin the width direction of the sheet P between the first roller pair andthe second roller pair is applied with the predetermined tension forcein the conveying direction over a range from the leading end of thetrailing end.

Then, it is determined whether the job is ended (S18-9), and in a casewhere the job is ended, the flow from S18-2 is repeatedly performedbased on the basis weight, the type, the image density, and the humidityinformation of the next job which is subsequently input. When the job isended, the driving motor M and the electromagnetic clutches CL2, CL3,and CL4 are turned off and the sheet passing is ended (S18-10), and thenthe job is ended (S18-11).

Similarly to the first embodiment, the setting values of the sheettension force in the embodiment are listed in FIG. 16. In the settingvalues of FIG. 16, as a result of repeatedly performing experimentsaccording to various conditions, it was confirmed that the ripplingamount in all the conditions was 1.0 mm or less similarly to the firstembodiment.

As described above, the sheet is coated with moisture to be humidifiedat a predetermined humidity or more and then passed in the sheet pullingand conveying apparatus where the center portion in the width directionof the sheet is extended in the conveying direction. Therefore, adifference in length in the conveying direction of the sheet in thecenter portion and the end portion in the width direction of the sheetis reduced, and thus the rippling can be improved.

Furthermore, it is possible to more effectively improve the rippling ofthe sheet by selecting (varying) the sheet tension force on extendingaccording to information (the basis weight, the type, the density, andthe environmental humidity) relating to the sheet.

Other Embodiments

In addition, the various conditions such as the basis weight, the type,the image density, and the environmental humidity as informationrelating to the sheet according to the first and second embodiments aregiven as an example, and thus other setting values may be used for themethod of setting the conditions. For example, as illustrated in FIG.19, in the configurations in the first and second embodiments, ahumidity detector 601 (for example, an infrared humidity measuringmeter) which detects a humidity contained in the sheet is provided onthe downstream side of the sheet humidifying apparatus 202 and on theupstream side of the sheet pulling and conveying apparatus 101. Then,the sheet tension force applied to the sheet may be changed according toa humidity of the sheet detected by the humidity detector. In this case,when the sheet contains a large amount of moisture, the tension forcebecomes small compared to the case of a less humidity.

Specifically, when the humidity of the sheet detected by the humiditydetector 601 is a second humidity larger than a first humidity, thecontroller 500C sets the load portion 131 and the torque limiters TL1 toTL3 with the second load smaller than the first load and applies thesecond tension force smaller than the first tension force to the sheet.

Further, the setting value of the tension force may be changed by thetype of sheet (coated paper, embossing paper, thin paper, recycledpaper, and the like) in the various conditions.

Further, the invention may be configured to change the load on the sheetbased on information relating to the sheet such as the informationrelating to at least one of the basis weight of sheet, the type ofsheet, the environmental humidity, the humidity contained in the sheet,and the image density on the sheet.

In the embodiment, the description has been made about the method ofinputting the basis weight of the sheet in the operation panel, but theinvention is not limited thereto. For example, a device such as anultrasonic sensor to detect the basis weight may be disposed on theconveying path of the sheet, and the load on the sheet may be changedbased on a detection value of the device. Further, a device such as anoptical sensor to detect the type of sheet may be disposed on theconveying path of the sheet, and the load on the sheet may be changedbased on a detection value of the device.

In addition, in the above-mentioned embodiment, the printer has beenexemplified as the image forming apparatus, but the invention is notlimited thereto. For example, another image forming apparatus such as acopying machine and a facsimile machine, or a multifunction peripheralcombined with the functions of these apparatus may be employed. Further,the image forming apparatus has been exemplified in which anintermediate transfer member is used, the toner images of the respectivecolors are transferred onto the intermediate transfer member in asequentially superimposed manner, and the toner images carried on theintermediate transfer member are transferred in a lump, but theinvention is not limited thereto. For example, an image formingapparatus may be employed in which a sheet bearing member is used, thetoner images of the respective colors are transferred onto the sheetcarried on the sheet bearing member in a sequentially superimposedmanner. When the invention is applied to the sheet rippling correctionapparatus used in these image forming apparatuses, the same advantagescan be obtained.

Further, the sheet rippling correction apparatus which is freelydetachable to the image forming apparatus has been exemplified in theembodiment described above, but the invention is not limited thereto.For example, the sheet rippling correction apparatus may be integrallyformed with the image forming apparatus, and when the invention isapplied to the sheet rippling correction apparatus, the same advantagecan be obtained.

Further, the sheet rippling correction apparatus (sheet processingapparatus) as an optional external apparatus which is freely detachableto the image forming apparatus has been exemplified in the embodimentdescribed above, but the invention is not limited thereto. For example,the sheet rippling correction apparatus may be integrally formed withthe image forming apparatus, and when the invention is applied to thesheet rippling correction apparatus, the same advantage can be obtainedas the entire image forming system. Further, the configuration that theoperation of the sheet rippling correction apparatus is controlled bycontrolling the controller included in the sheet rippling correctionapparatus using the controller included in the image forming apparatushas been exemplified, but the invention is not limited thereto. Forexample, only the image forming apparatus includes the controller, andthe operation of the sheet rippling correction apparatus may becontrolled by the controller included in the image forming apparatus.Even in such a configuration, the same advantage can be obtained.

Further, in the embodiment described above, the outer diameters of thefirst roller B105 and the second roller B107 are set such that one inthe center area in the width direction perpendicular to the conveyingdirection of the sheet is larger than that in the end area, but theinvention is not limited thereto. Any configuration may be employed aslong as the tension force in the conveying direction is applied to thecenter area in the width direction. Specifically, any configuration maybe employed as long as the outer diameter of at least one roller amongthe rollers included in the first roller pair and the second roller pairis provided to be larger than that of the center area in the widthdirection perpendicular to the conveying direction of the sheet.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2013-220889, filed Oct. 24, 2013, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A sheet conveying apparatus which conveys a sheetwith an image formed thereon by an image forming portion, comprising: aload portion which is configured to apply a tension force in a conveyingdirection to the sheet; and a load controller which performs control onthe load portion to vary a load thereof, wherein the load controllerperforms control on the load portion to vary the load thereof accordingto information relating to the sheet.
 2. The sheet conveying apparatusaccording to claim 1, further comprising: a first roller pair whichincludes a first roller A and a first roller B, wherein the first rollerB comes in press contact with the first roller A to form a nip portion,and nips and conveys the sheet; and a second roller pair which includesa second roller A and a second roller B and is positioned on adownstream side in the conveying direction of the first roller pair,wherein the second roller B comes in press contact with the secondroller A to form a nip portion, and nips and conveys the sheet, whereinan outer diameter of at least one of the first roller A, the firstroller B, the second roller A, and the second roller B in a center areain the width direction perpendicular to the conveying direction of thesheet is larger than that in an end portion, and wherein the loadportion applies a load onto a rotation of the first roller pair.
 3. Thesheet conveying apparatus according to claim 1, wherein the load portionincludes an electromagnetic brake of which the brake torque is changedby changing an exciting current, and wherein the load controllerperforms control on the load portion to vary the load by changing acurrent value excited in the load portion.
 4. The sheet conveyingapparatus according to claim 1, wherein the load portion includes aplurality of torque limiters and a plurality of clutches which connectsor disconnects the torque limiters, and wherein the load controllerperforms control such that the clutches are selectively connected ordisconnected to cause the torque limiters to vary the load.
 5. The sheetconveying apparatus according to claim 1, wherein the informationrelating to the sheet is information relating to at least one of a basisweight, a type of the sheet, an environmental humidity, a humiditycontained in the sheet, and an image density on the sheet.
 6. The sheetconveying apparatus according to claim 1, wherein the informationrelating to the sheet is input by an operation portion which is providedin the image forming portion.
 7. The sheet conveying apparatus accordingto claim 1, wherein when a basis weight of the sheet is a first basisweight, the load controller sets the load portion to be a first load,and wherein when the basis weight of the sheet is a second basis weightlarger than the first basis weight, the load controller sets the loadportion to be a second load larger than the first load.
 8. The sheetconveying apparatus according to claim 1, wherein when an image densityon the sheet is a first density, the load controller sets the loadportion to be a first load, and wherein when the image density on thesheet is a second density larger than the first density, the loadcontroller sets the load portion to be a second load larger than thefirst load.
 9. The sheet conveying apparatus according to claim 1,further comprising a humidity detection portion which detects anenvironmental humidity, wherein when the environmental humidity detectedby the humidity detection portion is a first humidity, the loadcontroller sets the load portion to be a first load, and wherein whenthe environmental humidity detected by the humidity detection portion isa second humidity lower than the first humidity, the load controllersets the load portion to be a second load larger than the first load.10. The sheet conveying apparatus according to claim 1, furthercomprising a moisture applying apparatus which applies moisture to thesheet on an upstream side in the conveying direction of the sheet fromthe load portion, wherein the load controller sets the load portion tobe a first load on the sheet of the first humidity, and wherein the loadcontroller sets the load portion to be a second load smaller than thefirst load on the sheet of which the humidity is a second humiditylarger than the first humidity.
 11. An image forming apparatuscomprising: an image forming portion which forms a toner image; a fixingportion which heats and fixes the toner image formed on a sheet by theimage forming portion; a load portion which applies a tension force in aconveying direction to the sheet passed through the fixing portion; anda load controller which performs control on the load portion to vary aload, wherein the load controller performs control the load of the loadportion according to information relating to the sheet.
 12. The imageforming apparatus according to claim 11, further comprising: a firstroller pair which includes a first roller A and a first roller B,wherein the first roller B comes in press contact with the first rollerA to form a nip portion, and nips and conveys the sheet; and a secondroller pair which includes a second roller A and a second roller B andis positioned on a downstream side in the conveying direction of thefirst roller pair, wherein the second roller B comes in press contactwith the second roller A to form a nip portion, and nips and conveys thesheet, wherein an outer diameter of at least one of the first roller A,the first roller B, the second roller A, and the second roller B in acenter area in the width direction perpendicular to the conveyingdirection of the sheet is larger than that in an end portion, andwherein the load portion applies a load onto a rotation of the firstroller pair.
 13. The image forming apparatus according to claim 11,wherein wherein the load portion includes an electromagnetic brake ofwhich the brake torque is changed by changing an exciting current, andwherein the load controller performs control on the load portion to varythe load by changing a current value excited in the load portion. 14.The image forming apparatus according to claim 11, wherein wherein theload portion includes a plurality of torque limiters and a plurality ofclutches which connects or disconnects the torque limiters, and whereinthe load controller performs control such that the clutches areselectively connected or disconnected to cause the torque limiters tovary the load.
 15. The image forming apparatus according to claim 11,wherein wherein the information relating to the sheet is informationrelating to at least one of a basis weight, a type of the sheet, anenvironmental humidity, a humidity contained in the sheet, and an imagedensity on the sheet.
 16. The image forming apparatus according to claim15, further comprising an operation portion, wherein the informationrelating to the sheet is input by the operation portion.
 17. The imageforming apparatus according to claim 11, wherein wherein when a basisweight of the sheet is a first basis weight, the load controller setsthe load portion to be a first load, and wherein when the basis weightof the sheet is a second basis weight larger than the first basisweight, the load controller sets the load portion to be a second loadlarger than the first load.
 18. The image forming apparatus according toclaim 11, wherein wherein when an image density on the sheet is a firstdensity, the load controller sets the load portion to be a first load,and wherein when the image density on the sheet is a second densitylarger than the first density, the load controller sets the load portionto be a second load larger than the first load.
 19. The image formingapparatus according to claim 11, further comprising a humidity detectionportion which detects an environmental humidity, wherein when theenvironmental humidity detected by the humidity detection portion is afirst humidity, the load controller sets the load portion to be a firstload, and wherein when the environmental humidity detected by thehumidity detection portion is a second humidity lower than the firsthumidity, the load controller sets the load portion to be a second loadlarger than the first load.
 20. The image forming apparatus according toclaim 11, further comprising a moisture applying apparatus which appliesmoisture to the sheet on an upstream side in the conveying direction ofthe sheet from the load portion, wherein the load controller sets theload portion to be a first load on the sheet of the first humidity, andwherein the load controller sets the load portion to be a second loadsmaller than the first load on the sheet of which the humidity is asecond humidity larger than the first humidity.